In a fuel system having hydraulically-actuated electronically controlled unit injectors (HEUI), high pressure hydraulic actuating fluid flows into a chamber, located within the injector, and pushes down on a plunger which pushes fuel out from a plunger cavity, and out the injector through a nozzle. A solenoid, located within the injector, controls when the high pressure actuating fluid is exposed to the plunger by moving a poppet valve. The amount of fuel injected is controlled by adjusting the duration the solenoid is on.
The viscosity of the actuating fluid affects both the amount of fuel delivered by the injector, and when the delivery process begins. For example, in cold temperatures the actuating fluid is thicker (more viscous) than at warm temperatures. Therefore, when an electrical signal is delivered to a solenoid, commanding the solenoid to deliver actuating fluid to the injector, the fluid flows at a slower rate into the chamber to push against the plunger. With the actuating fluid moving at a slower rate there is an increased delay before the injector begins delivering fuel. Furthermore, when the solenoid is turned off to stop delivery of the fuel, the reduced flow rate of the actuating fluid results in less total fuel being injected between when the solenoid is turned on and off. Hence, with a high viscous actuating fluid seen at cold starting temperatures as compared to higher temperature operating conditions, less fuel is delivered by the injectors and the fuel is delivered later in the crank cycle. Under these conditions, overall engine performance is adversely effected, resulting in incomplete combustion, low power, white smoke, etc.
The viscosity of the actuating fluid is a function of the fluid type, the amount the fluid is sheared from the transmission through the hydraulic circuit, and the temperature of the fluid. In an operating engine, neither the type of fluid, nor the temperature is fixed. The fuel system may use a variety of actuation fluids. For example, a more viscous SAE 15W40 engine oil or a less viscous 0W20 engine oil may be used. Also, the fuel system operates over a wide range of temperatures, e.g., -50 degrees Fahrenheit through 250 degrees Fahrenheit.
The reduction in fuel delivery and delays in fuel delivery increase as the viscosity of the actuating fluid increases. If the changes in viscosity are not accounted for, the fuel delivery and timing may be incorrect making it difficult to start and run the engine especially at high viscosities encountered at cold temperatures. If the fuel delivery is too small, the engine may not start or be underpowered. If the fuel delivery is too large, the engine structural capabilities may be exceeded, or excessive smoke may be produced. Misfire may occur due to fuel delivery at incorrect (late) ignition timings.
The present invention is directed to overcoming one or more of the problems identified above.